Project Summary Formation of complex neural circuits must be precisely controlled during the development of the nervous system. Axons navigate to their targets by detecting both long-range and short-range signals presented by cells in the environment that can act as attractants or repellants. Defects in axon guidance and targeting can result in a number of neurodevelopmental disorders, such as epilepsy, schizophrenia, Autism Spectrum Disorders, and intelligence disabilities. The Drosophila visual system is an excellent model system for studying the basic mechanisms of axon guidance and neural circuit formation. The terminals of R7 and R8 photoreceptors, responsible for different aspects of color vision, are segregated into distinct target layers of the medulla, a central region of visual processing in the brain. Photoreceptors also must project in a retinotopic manner to maintain a precise representation of the visual field within the brain. In a screen for molecules that control color photoreceptor axon targeting, knocking down plexin A (plexA) in the brain using RNAi caused R7 photoreceptors to prematurely terminate in the R8 target layer of the medulla and fail to expand their axon terminals. A protein null mutation in plexA (plexAMb09499) caused by a Minos element insertion in a coding exon has the same effect on R7 axons. plexA is required in neurons, but not in photoreceptors, for R7 to reach the correct target layer. PlexA protein is expressed close to this target layer, suggesting that it may act as an attractive signal for R7 axons, in contrast to its known role in repelling L3 lamina neuron axons from this layer. Mosaic analysis will be used to confirm that this is the site of PlexA action. Overexpression of PlexA or a truncated form lacking the cytoplasmic domain in photoreceptors results in hyperfasciculation and premature termination of their axons. This misexpression phenotype will be used to characterize the mechanism of PlexA action and test the hypothesis that PlexA acts as an attractive or adhesive signal for photoreceptors. The known receptor and ligand for PlexA, Semaphorin- 1a, is not required for normal targeting of R7 photoreceptors. We will investigate whether PlexA instead interacts with an uncharacterized CUB domain protein, CG6024; knocking down CG6024 in photoreceptors using RNAi also disrupts R7 targeting. Homologs of PlexA have been studied in the organization of the vertebrate retina, and Plexin family members also participate in regulation of the immune response, heart and vessel formation, and even the progression of cancer. Preliminary results suggest that PlexA controls R7 targeting through a novel signaling mechanism, distinct from those previously described. Therefore, this work may provide new insight into the mechanisms that control a variety of developmental and disease processes in higher organisms.